This invention relates to downhole tools used to acquire and test a sample of fluid from a formation. More particularly, this invention relates to a sealing arrangement that creates a seal between a sample probe and a formation in order to isolate the probe from wellbore fluids.
Formation testing tools are used to acquire a sample of fluid from a subterranean formation. This sample of fluid can then be analyzed to determine important information regarding the formation and the formation fluid contained within, such as pressure, permeability, and composition. The acquisition of accurate data from the wellbore is critical to the optimization of hydrocarbon wells. This wellbore data can be used to determine the location and quality of hydrocarbon reserves, whether the reserves can be produced through the wellbore, and for well control during drilling operations.
Formation testing tools may be used in conjunction with wireline logging operations or as a component of a logging-while-drilling (LWD) or measurement-while-drilling (MWD) package. In wireline logging operations, the drill string is removed from the wellbore and measurement tools are lowered into the wellbore using a heavy cable (wireline) that includes wires for providing power and control from the surface. In LWD and MWD operations, the measurement tools are integrated into the drill string and are ordinarily powered by batteries and controlled by either on-board or remote control systems.
To understand the mechanics of formation testing, it is important to first understand how hydrocarbons are stored in subterranean formations. Hydrocarbons are not typically located in large underground pools, but are instead found within very small holes, or pores, within certain types of rock. The ability of a formation to allow hydrocarbons to move between the pores, and consequently into a wellbore, is known as permeability. Similarly, the hydrocarbons contained within these formations are usually under pressure and it is important to determine the magnitude of that pressure in order to safely and efficiently produce the well.
During drilling operations, a wellbore is typically filled with a drilling fluid (xe2x80x9cmudxe2x80x9d), such as water, or a water-based or oil-based mud. The density of the drilling fluid can be increased by adding special solids that are suspended in the mud. Increasing the density of the drilling fluid increases the hydrostatic pressure that helps maintain the integrity of the wellbore a and prevents unwanted formation fluids from entering the wellbore. The drilling fluid is continuously circulated during drilling operations. Over time, as some of the liquid portion of the mud flows into the formation, solids in the mud are deposited on the inner wall of the wellbore to form a mudcake.
The mudcake acts as a membrane between the wellbore, which is filled with drilling fluid, and the hydrocarbon formation. The mudcake also limits the migration of drilling fluids from the area of high hydrostatic pressure in the wellbore to the relatively low-pressure formation. Mudcakes typically range from about 0.25 to 0.5 inch thick, and polymeric mudcakes are often about 0.1 inch thick. The thickness of a mudcake is generally dependent on the time the borehole is exposed to drilling fluid. Thus, in MWD and LWD applications, where a section of the borehole may be very recently drilled, the mudcake may be thinner than in wireline applications.
The structure and operation of a generic formation tester are best explained by referring to FIG. 1. In a typical formation testing operation, a formation tester 100 is lowered to a desired depth within a wellbore 102. The wellbore 102 is filled with mud 104, and the wall of wellbore 102 is coated with a mudcake 106. Once formation tester 100 is at the desired depth, it is set in place by extending a pair of feet 108 and an isolation pad 110 to engage the mudcake 106. Isolation pad 110 seals against mudcake 106 and around hollow probe 112, which places internal cavity 119 in fluid communication with formation 122. This creates a fluid pathway that allows formation fluid to flow between formation 122 and formation tester 100 while isolated from wellbore fluid 104.
In order to acquire a useful sample, probe 112 must stay isolated from the relative high pressure of wellbore fluid 104. Therefore, the integrity of the seal that is formed by isolation pad 110 is critical to the performance of the tool. If wellbore fluid 104 is allowed to leak into the collected formation fluids, an non-representative sample will be obtained and the test will have to be repeated.
Isolation pads that are used with wireline formation testers are generally simple rubber pads affixed to the end of the extending sample probe. The rubber is normally affixed to a metallic plate that provides support to the rubber as well as a connection to the probe. These rubber pads are often molded to fit with the specific diameter hole in which they will be operating. These types of isolator pads are commonly molded to have a contacting surface that is cylindrical or spherical.
While conventional rubber pads are reasonably effective in some wireline operations, when a formation tester is used in a MWD or LWD application, they have not performed as desired. Failure of conventional rubber pads has also been a concern in wireline applications that may require the performance of a large number of formation pressure tests during a single run into the wellbore, especially in wells having particularly harsh operating conditions. In a MWD or LWD environment, the formation tester is integrated into the drill string and is thus subjected to the harsh downhole environment for a much longer period than in a wireline testing application. In addition, during drilling, the formation tester is constantly rotated with the drill string and may contact the side of the wellbore and damage any exposed isolator pads. The pads may also be damaged during drilling by the drill cuttings that are being circulated through the wellbore by the drilling fluid.
Therefore, there remains a need in the art to develop an isolation pad that provides reliable sealing performance with an increased durability and resistance to damage. Therefore, the present invention is directed to methods and apparatus for isolator pad assemblies that effectively seal against a wellbore and are resistant to damage typically incurred during drilling operations. It is also an object of the present invention to provide an isolator pad assembly that has an extended life so as to enhance the number of tests that can be performed without replacing the pad.
Accordingly, there are provided herein methods and apparatus for isolator pad assemblies that comprise a primarily metallic pad member and a retractable resilient sealing member. The resilient sealing member is maintained in a retracted, protected position until extended to seal against the wellbore. Once extended to a sealing position, the resilient sealing member acts as a primary seal while the metallic pad member acts as a secondary seal.
One embodiment of a preferred isolator pad comprises a cylindrical outer sleeve that is sealingly engaged with a tool body and is capable of lateral translation in respect to the tool body. Affixed to the extending end of the outer sleeve is a metallic pad that has a contacting surface that is curved and preferably has a raised lip surrounding a penetration through the pad. An inner sleeve is slidingly engaged within the penetration through the pad and has a resilient ring molded to one end. The inner sleeve has an extended position wherein the resilient ring extends past the outer surface of the pad and a retracted position where the resilient ring does not extend past the surface of the pad.
Once the formation testing tool reaches the desired location in the wellbore, the tool is activated and the outer sleeve extended. The metallic pad engages the mudcake on the wellbore and compresses the mudcake until the raised lip contacts the formation. Once the outer sleeve and pad are extended, the inner sleeve extends so that the resilient ring contacts the mudcake. The contact between the resilient ring and the mudcake forms a primary seal to prevent wellbore fluids from entering the inner sleeve during a formation test. A secondary seal is formed by the metallic pad compressing the mudcake.